1. Field of the Invention
The present invention relates generally to antenna measurement systems, and particularly to a system for estimating antenna patterns using far field samples.
2. Technical Background
An antenna is an electrical transducer that converts an electrical signal into a radio frequency (RF) signal, and vice versa. An antenna is used in conjunction with a radio transmitter or radio receiver. For example, when a radio station transmits a signal (e.g., music), the signal is provided to a radio transmitter that amplifies and conditions the signal before providing it to the antenna. The transmitter “excites” that antenna such that the signal is converted or transduced into RF energy that propagates through space. On the receive side, an antenna intercepts some of the RF energy propagating through space and converts that energy into a very small signal that is supplied to the receiver. The receiver amplifies the small signal and conditions it for subsequent use. Thus, every RF system must have an antenna if it is to be functional. Stated differently, a radio broadcast system, television broadcast system, a cell phone, radar, or a Bluetooth device, to name a few, would be inoperable without an antenna.
There a various types of antennas that are commonly used in the art depending on a variety of factors. In general, an antenna typically includes a plurality of “antenna elements” that are configured to convert electrical signals into RF energy, or vice-versa. These “elements” are electrical conductors that are coupled to the receiver (or transmitter) by transmission lines. Stated differently, the antenna elements may be arranged and configured to form various types of antennas such as, e.g., horn antennas, reflective dish antennas (e.g., a parabolic reflector dish), a phased array antenna (i.e., an array of elements that are excited by electrical signals having a variety of phases), and the like.
One reason why a person skilled in the art would select one antenna over another antenna relates to the shape and direction of the radio waves that are emitted by the selected antenna. The shape and direction of the radio waves emitted by the antenna is often referred to as the “antenna pattern.” Omnidirectional antennas transmit the RF energy equally in all directions whereas “directional” or “high gain” antennas transmit the RF energy in a particular direction.
There are two primary aspects of an antenna that have to be in working order for the antenna to properly do its job. The first aspect relates to the physical condition of the antenna: the operability and position the antenna elements (e.g., feed elements, reflectors, etc.) have to be correct. In other words, if a part of the antenna is broken, the antenna will not perform correctly. The second aspect relates to the electrical signals that are being supplied to the antenna. The electrical signals must be calibrated such that they properly “excite” the antenna. As before, if the electrical signals are not calibrated properly, the antenna will not work properly.
In order to determine if a given antenna is working properly, antenna engineers typically place the antenna in an anechoic chamber or outdoor far-field range, excite it with the electrical signals, and measure the resultant antenna pattern. Thus, the antenna pattern can be used to determine if either or both of the two aspects (i.e., physical condition of the antenna or electrical excitations) are defective. One of the drawbacks associated with this approach relates to previously installed antennas, or antennas that are “in the field.” For the most part, these antennas have to be removed and transported to the proper facility for the aforementioned testing and repair. This result is both costly and inefficient.
In one approach, the recovery of an antenna's excitation from a limited number of pattern measurements and subsequent prediction of additional portions of the pattern has been considered. In this approach, the antenna is scanned in its near field. After careful calibration for mutual coupling effects, near field scanners obtain a very detailed model of the antenna's current distribution. From this current distribution, accurate models of the resulting antenna pattern can be predicted. One disadvantage of the near-field scanning approach is that sophisticated, expensive equipment and careful calibration are required. It should be noted that if the scanner is designed to fit exactly one specific antenna, some equipment complexity can be removed. However, this presents another drawback in that this system must be specifically tailored to the specified antenna. What is needed is an approach that is general enough to handle many different antenna types essentially without modification, can use simple equipment (e.g., such as a field strength measurement meter), and has minimal calibration requirements. The measurements should be taken in the far field so that mutual coupling is substantially eliminated.
In another approach, far-to-near field transformations are used to filter far-field measurements. Because this approach works exclusively with planar antennas, any excitations found to be off of the plane constitute error, and are removed. In essence, this approach manipulates the excitation structure of the antenna under the assumption of a known configuration. The drawback to this approach is that the algorithm is therefore linear. What is needed is a non-linear optimization that allows antenna configurations to vary.
What is needed, therefore, is a system and method for testing, calibrating and repairing an antenna in the field. The needed system and method would be able to estimate or extrapolate the antenna pattern using a relatively small number of far field measurements and provide data that can be used to diagnose the antenna malfunction.